1. Technical Field
The invention relates to an optical projection system with a light source and a device for shuttering the light stream of the light source.
2. Background Art
Such projection systems have, especially in the case of high-powered projectors, a light source which is not or is only insufficiently regulable as to the strength of the light stream.
While in conventional projectors equipped with halogen lamps, for example, the light stream of the halogen lamp can be regulated electronically, it is necessary in the case of the metal vapor lamps or xenon high-pressure lamps used for high-powered projectors because of the greater light intensity. It is necessary to install a mechanically working shuttering device in the path of the light beam. For this purpose, it has been proposed, for instance, that the light stream be influenced in the zone of the object (focus) of the projector by a mechanically adjustable Iris shutter. The disadvantage of this arrangement consists, on the one hand, of the damming up of heat generated thereby in the projection object, and on the other hand, of the mechanical complexity of the arrangement.
Alternative continuous shuttering devices are known so-called "gray" or "neutral" wedges which can be rotated or pushed along. In these, a transparent plate is colored increasingly gray in a working (movement) direction, so that in the direction of work it passes in succession from full transparency (no coloring) to complete opacity (completely black), preferably continuously. These gray wedges are arranged as elongated plates or rotatable discs in the path of the beam after the optic projection. The disadvantages of these gray wedges consist, among others, of the fact that they require much space, their direction of movement is mechanically fallible, their maximum speed of movement or maximum frequency with many rapidly varying light-dark changes is relatively small, and the illumination of the object (image) is uneven. This latter occurs because, with halfway tolerable size of the gray wedge, the lighted field is darker on one edge than on the opposite edge. Iris shutters arranged in the focus of projection and gray wedges have in common the further disadvantage that they only provide the desired power in cooperation with a very definite projection object in each case (burning width, lens diameter, etc.), that is, the best possible results for the particular shuttering device used in each case.
As another alternative shuttering device, Venetian louvre type arrangements have been proposed with a plurality of parallel, narrow louvre-type shutters which can swing in common around their longitudinal axes for arrangement in greater cross-sections of light stream. Such devices are less slow, it is true, but extremely sensitive mechanically.
From U.S. Pat. No. 4,668,077 is known an optical projection apparatus for the projecting of patterns for photomasks in which a mercury high-pressure vapor lamp is provided with an elliptical reflector, so that the cone of light produced is first constricted. At the point of constriction, a rotatable shutter projected with openings is placed in the path of the light beam by which the light beam is fully passed through or fully shuttered in strokes. A continuous regulation of brightness of the stream of light is not possible with this.
From German publication 3,203,800 Al is known a test projector for color TV cameras in which a mechanically controlled or regulated shutter is arranged in the cone of light between the lamp provided with a reflector and a first constriction of the light cone. With this shuttering device, the stream of light can be shuttered or released between about 0 and 100 %. In the different shuttering steps, however, the cone of light is influenced asymmetrically which acts against a uniform lighting of the object. To solve this problem in this own arrangement, a light mixing chamber, also known as a diffusor, is arranged in the first constriction point after the lamp. Through this, the asymmetry of the cone of light passing through the shuttering device is equalized. Such diffusors are unsuitable for high-powered projectors because of too great loss of light.
From German publication 1,288,907 is known a shutter for photographic and movie cameras (and thus not for projectors) in which the light stream is generally shuttered by two stationary side-shutter parts to a permanent part. The part of the light cone-field remaining between these two stationary side shutters can be shuttered continuously by a single wedge-shaped movable shutter. For high-powered projectors, the general shuttering out of a part of the light stream is undesirable. The single wedge-shaped shutter used leads also, in some positions, to a non-symmetrical lighting of the object. Moreover, in zones of slight shuttering effect, it has a similar effect to that of the Iris shutter, that is, the available light cone is constricted from the edge which leads to a vignetting of the picture field.